1. Field of the Invention
This invention relates to a water dispersable coating composition based on an epoxy resin. Epoxy resins are capable of forming tough coatings which are highly resistant to hydrolysis and to chemical attack. Coatings exhibiting particularly high resistance to hydrolysis are for example required for internal coatings for food and beverage cans which may have to withstand heating in contact with acidic aqueous liquids such as fruit juice or beer. The coating should not hydrolyse to release any substances which might impair the flavour of the food or beverage and should adhere sufficiently strongly to the can, which is usually of tinplate or aluminium, to withstand the conditions imposed during pasteurization of the food or beverage.
It is, however, difficult to produce coating compositions containing epoxy resin in a water dispersable form. Coatings based on volatile organic solvents are being regarded with increasing disfavour as the evaporated solvent causes atmospheric pollution and the cost of organic solvent is rapidly increasing. The present invention seeks to prepare a coating composition based on an epoxy resin which can be applied to a substrate from a solution or stable dispersion in a diluent consisting mainly of water.
2. Description of Prior Art
European Patent Publications Nos. 6334 and 6336 describe a process for preparing a self-emulsifiable epoxy ester copolymer mixture which comprises esterifying a solvent-soluble carboxyl-functional polymer with an epoxy resin which is a mixture comprising aromatic polyethers containing oxirane functionality, at least 5% of the aromatic polyethers being constituted by diepoxide, the mixture of aromatic polyethers constituting at least 40% of the total resin solids content and providing oxirane functionality in a stoichiometric deficiency with respect to carboxyl functionality in the carboxyl-functional polymer of from 1:2 to 1:20, the esterification being carried out in organic solvent medium in the presence of more than 2% of an amine esterification catalyst, based on the weight of the reactants subjected to esterification, and at an elevated temperature sufficient to cause the oxirane functionality to react with the carboxyl functionality until the oxirane functionality is substantially entirely consumed, and reacting at least a portion of the carboxyl functionality in the resultant polymer-epoxy resin hydroxy ester reaction product with a base to render the reaction product self-emulsifiable in water. European Patent Publication No. 6334 teaches that to avoid gelation of the reaction mixture a high proportion of the amine esterification catalyst, such as 4-10% by weight based on the reactants, must be used. British Patent Publication No. 2,059,968 describes an alternative method for preparing a water dispersable coating composition in which an epoxy resin is reacted with a solution, in a mixture of water and an organic solvent, of a carboxyl-functional acrylic polymer and an amine. In this process also, a high proportion of amine is present during the reaction of the epoxy resin and the carboxyl-functional polymer. An alternative process for preparing a water dispersable coating composition based on an epoxy resin and an addition polymer containing carboxylic acid groups is described in U.S. Pat. No. 4,212,781. In this process the epoxy resin is reacted with addition polymerisable material at least part of which is carboxylic acid functional in the presence of 6-7% by weight of benzoyl peroxide catalyst. The present invention prepares a water dispersable coating composition from an epoxy resin without requiring the use of such high levels of catalyst.